In recent years, as a variety of electronic equipments become compact, electronic devices to be installed inside the electronic equipments have become more compact and higher in performance. As one of the electronic devices, there is a ceramic electronic device, such as a CR built-in substrate and a multilayer ceramic capacitor, and the ceramic electronic devices have been required to be more compact and higher in performance.
To pursue a more compact ceramic electronic device having a higher capacity, there is a strong demand for making a dielectric layer thinner. Recently, a thickness of a dielectric green sheet composing a dielectric layer has become a several μm or less.
To produce a ceramic green sheet, normally, a ceramic coating material composed of ceramic powder, a binder (an acrylic based resin and a butyral based resin, etc.), a plasticizer (phthalate esters, glycols, adipic acids, and phosphoric esters) and an organic solvent (toluene, MEK and acetone, etc.). Next, the ceramic coating material coated on a carrier sheet (a supporting body made by PET and PP) by using the doctor blade method, etc. and dried by heating.
Also, a method of producing by preparing a ceramic suspension wherein the ceramic powder and binder are mixed in a solvent, then, biaxial stretching a film-shaped molded item obtained by molding the suspension has been considered in recent years.
A method of producing a multilayer ceramic capacitor by using the ceramic green sheet explained above will be explained in detail. An internal electrode conductive paste containing metal powder and a binder is printed to be a predetermined pattern on the ceramic green sheet and dried to form an internal electrode pattern. After that, the green sheet is peeled from the carrier sheet and stacked by a predetermined number of layers. Here, two methods are proposed, that are a method of peeling the green sheet from the carrier sheet before stacking in layers and a method of peeling the carrier sheet after stacking in layers and adhering by compression, but the difference is not large. Finally, the stacked body is cut to be chips, so that green chips are prepared. After firing the green chips, external electrodes are formed, so that a multilayer ceramic capacitor and other electronic devices are produced.
When producing a multilayer ceramic capacitor, an interlayer thickness of sheets formed with internal electrodes is in a range of 3 μm to 100 μm or so based on a desired capacitance required as a capacitor. Also, in a multilayer ceramic capacitor, a part not formed with internal electrodes is formed on an outer part in the stacking direction of the capacitor chip.
In such a multilayer ceramic capacitor, a polyvinyl butyral resin having a polymerization degree of 1000 or less (Mw=50,000) is used as a binder in some cases (refer to the Japanese Patent Publication No. 10-67567). However, in the case of using a normal polyvinyl butyral resin as a binder, there are problems that adhesiveness declines and stacking becomes difficult when a thickness of the green sheet becomes thin.
In recent years, as electronic equipments become more compact, electronic devices to be used therein have rapidly become more compact. In multilayer electronic devices as typified by a multilayer ceramic capacitor, rapid development has been made on increasing the number of layers to be stacked and attaining a thinner interlayer thickness. To respond to the technical trends, a thickness of a green sheet, which determines the interlayer thickness, has almost become 3 μm or less to 2 μm or less. Therefore, in a production process of a multilayer ceramic capacitor, it is necessary to handle extremely thin green sheets and to design very advanced green sheet properties.
As characteristics required as the properties of such an extremely thin green sheet, sheet strength, flexibility, smoothness, adhesiveness when being stacked, handlability (electrostatic property), etc. may be mentioned, and balance of a higher order is required.
Note that, as shown in the Japanese Unexamined Patent Publication No. 6-206756, there is known a technique of using a polyvinyl butyral resin having a polymerization degree of 1000 or more as a binder in green sheet slurry containing an aqueous solvent for a purpose of eliminating a short-circuiting defect.
However, the Japanese Unexamined Patent Publication No. 6-206756 is not for particularly attaining a thinner organic solvent based green sheet, and it also has problems that the adhesiveness declines and stacking becomes difficult when a thickness of the green sheet is made thin.
Also, as a method of improving adhesiveness of the green sheet, as shown in the Japanese Unexamined Patent Publication No. 5-279108, there is known a method of making a hot-melt adhesive, such as rosin, contained in dielectric slurry. However, in this method, adhesiveness at a high temperature can be expected but it does not lead to an improvement of adhesive strength at a normal temperature and, when the dielectric layer is made thin, adhesive strength at a normal temperature is insufficient and stacking is not easy.
Furthermore, as disclosed in the Japanese Unexamined Patent Publication No. 7-99132, a method of using a water-soluble polyvinyl acetal resin as a binder resin and adding amines as a plasticizer is proposed. However, in this method, the binder resin and the plasticizer are selected only considering the adhesiveness, and other sheet properties, such as tensile strength and surface roughness, may decline when the dielectric layer is made thin.
Furthermore, as disclosed in the Japanese Unexamined Patent Publication No. 10-166343, there is provided a method of applying to a surface of the green sheet a self-adhesion inductive liquid for dissolving or swelling a binder resin in a green sheet. However, in this method, a step of applying the liquid on the surface of the produced green sheet is furthermore added, so that the procedure becomes cumbersome and complicated.
Also, as disclosed in the Japanese Unexamined Patent Publication No. 13-106580, a method of using a polyvinyl acetal resin as a binder resin and adding phthalate ester based, glycol based and amino alcohol based plasticizers as a plasticizer is proposed. However, in this method, a plurality of kinds of plasticizers are used and other sheet properties, such as tensile strength and surface roughness, decline when the dielectric layer is made thin.
Furthermore, as disclosed in the Japanese Unexamined Patent Publication No. 14-104878, a method of mixing a plurality of kinds of polyvinyl acetal resins as a binder is proposed. However, in this method, mixing of resin kinds is performed by only considering an improvement of the adhesiveness, and other sheet properties, such as tensile strength and surface roughness, decline when the dielectric layer is made thin.